Vibrio vulnificus and V. parahaemolyticus are among the most significant marine pathogens and dominate seafood-related illness in temperate regions. Here, an integrated, interdisciplinary study is proposed using these vibrios as model organisms to (i) explore the environmental forcings (physical and chemical), which lead to abundance of pathogenic bacteria indigenous to the marine environment on different temporal and spatial scales (Aims 1 and 2), and (ii) determine bounds of genetic diversity, which define ecological and evolutionary units in co-existing populations of pathogenic bacteria (Aims 3 and 4). Mt Hope Bay, a partially-mixed, temperate estuary in Narragansett Bay, Massachusetts, is proposed as an excellent study site. It contains both anthropogenically influenced and more pdstJne locations, represents (due to extensive thermal pollution) a good model for exploration of the effect of climate change on pathogen populations, and allows effective integration with Project 3 and the University of Massachusetts Mt Hope Bay Natural Laboratory. The combined expertise of environmental physics, microbiology and genomics make the Project 4 team well-qualified for advancing our understanding of what drives the growth and emergence of pathogenic bacteria in the natural environment. The overall significance and broader impact of the proposed study lie in the establishment of a model for (i) pathogen ecology and evolution, (ii) risk assessment of pathogen emergence, and (iii) the extent to which observed genomic variants represent ecological and evolutionary units that can be seen as the bacterial equivalent to the eukaryotic sexual species.